Method of carburizing and quenching a steel member

ABSTRACT

A method of carburizing and quenching a steel member includes: a reduced pressure carburization step in which a steel member is contacted with carburization gas under reduced pressure, a slow cooling step in which the steel member is then slowly cooled in a cooling gas, and a quenching step of heating a selected portion of the cooled steel member using high-density energy and subsequently subjecting the selected portion to rapid cooling. The steel member subjected to the low-pressure carburization step includes a first portion in which a diffusion rate of carbon taken therein during carburization is high because of its shape and a second portion in which the diffusion rate of carbon is lower than that of the first portion. The reduced-pressure carburization step is controlled to give a carbon concentration at the surface of the first portion in a range of 0.65±0.1 weight % after diffusion.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2008-115024 filed on Apr. 25, 2008 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a method of carburizing and quenching (“carburizing-quenching”) a steel member, for example a gear, wherein the steel member has a first portion in which the diffusion rate of the carbon introduced during carburization is high because of the shape of the member and a second portion in which the diffusion rate of the introduced carbon is lower than in the first portion.

DESCRIPTION OF THE RELATED ART

A steel member such as a gear is typically subjected to a carburizing-quenching process in order to improve its surface hardness while maintaining its toughness. The carburizing-quenching process is a process in which a steel member is subjected to carburization to increase the carbon concentration in a surface region of the member while heating the steel member at a temperature equal to or higher than its austenitizing temperature and subsequently quenching to secure toughness of a core region of the member and the surface hardness.

One gas carburizing method is a continuous gas carburizing method conducted in a large thermal processing furnace with an oil quenching bath at its outlet. The carburization is conducted over a long period, immediately followed by oil quenching. In gas carburizing, carbon diffuses into a steel member by an equilibration reaction, thus requiring the steel member to be exposed to a carburizing gas atmosphere at a high temperature for a long period of time. Oil is used as a coolant for quenching to suppress distortion by relatively slower cooling as compared to water cooling. However, in oil quenching, where a large number of steel members are simultaneously immersed in an oil bath, distortion is caused within each single steel member because of a quenching time difference as between that part of the steel member which first enters oil and the remaining portion which follows into the oil. Thus, in spite of the use of oil for quenching, it is difficult to solve the problem of distortion in steel members obtained by the above-described carburizing-quenching process. Furthermore, a variation in the quenching quality also occurs among steel members depending on the positions in which the steel members are placed.

Furthermore, because the related-art carburizing-quenching process requires prolonged carburization in a large furnace as described above, it consumes a large amount of energy. Accordingly, a need exists to shorten the time required for the carburizing-quenching process, to reduce the consumption of energy, and to downsize the carburizing-quenching equipment.

The use of reduced-pressure carburization (vacuum carburization) is considered to be effective in reducing the consumption of energy in the carburizing-quenching process.

With regard to quenching after carburization, a high-frequency quenching method has been suggested in which a component is subjected to localized quenching instead of simultaneously quenching the whole body of the component (see Japanese Patent Application Publication No. JP-A-11-131133).

SUMMARY OF THE INVENTION

The inventors of the present invention have now identified disadvantages in using the reduced-pressure carburization method. In the case of the conventional gas carburizing method, the carburizing is through an equilibration reaction and thus a carbon potential can be calculated in advance to set conditions. In reduced-pressure carburization, however, setting of such conditions is difficult because a non-equilibrium reaction is used. The present inventors also found that, when a steel member having a toothed surface, such as a gear, is subjected to reduced-pressure carburization, diffusion rates of the introduced carbon differ for different portions of the gear. Thus, the resultant surfaces have different carbon concentrations depending on shape and thus a desired effect may not be provided at a portion that should be surface modified by carburization.

Accordingly, it is an objective of the present invention to provide a method for treating a steel member by which a steel member, having first and second carbon diffusion rates in different portions, can be subjected to a reduced-pressure carburization under optimal conditions.

The present invention provides a method of manufacturing a steel member that includes: a reduced-pressure carburization step of carburizing a steel member in carburization gas under a reduced pressure, a slow cooling step in which the steel member obtained through the reduced-pressure carburization step is slowly cooled in cooling gas, and a quenching step in which a desired portion of the cooled steel member is heated using high-density energy and subsequently subjected to rapid cooling. The steel member subjected to the reduced-pressure carburization has a shape including a first portion in which the diffusion rate of carbon introduced during carburization is high, and a second portion in which the diffusion rate of the introduced carbon is lower than that in the first portion. The reduced-pressure carburization step is performed under conditions regulated to give a carbon concentration at the surface of the first portion in a range of 0.65±0.1 weight % after diffusion.

The method according to the present invention includes a reduced-pressure carburization step and a quenching step in which the steel member is heated by high-density energy and subsequently rapidly cooled, and also an intermediate slow cooling step. By this method, the steel member can be carburized-quenched equal to or better than in related art, while distortion is significantly suppressed. The process time is also significantly shorter than in the related art. Thus, the amount of energy used and the cost are significantly reduced.

As noted above, the reduced-pressure carburization of the present invention is performed under conditions giving a carbon concentration at the surface of the first (“easy carbon diffusion”) portion in a range of 0.65±0.1 weight % after diffusion.

Further, the reduced-pressure carburization of the present invention, under the conditions described above, results in a second (“difficult carbon diffusion”) portion, which has a carbon diffusion rate lower than that of the first portion, and which has a surface with a carbon concentration higher than that of the first portion and equal to or lower than 0.85 weight after diffusion. Accordingly, the overall carbon concentration of substantially the entire surface of the steel member is within a range from 0.55 to 0.85 weight % after diffusion. By limiting the carbon concentration at the surface within this range, the steel member can be subjected to the subsequent special quenching step in which the member is selectively heated with high-frequency energy and is subsequently subjected to rapid cooling. As a result, even in a portion whose surface carbon concentration is close to the lower limit (first portion), the quenching effect will be sufficient. At the same time, in the second portion having a carbon concentration at its surface close to the upper limit (“difficult carbon diffusion portion”), formation of cementite due to excessive carbon is reduced. Thus, a superior modified surface can be obtained by quenching.

The carburization conditions as described above must be determined by performing a plurality of preliminary experiments with different temperatures, types of carburization gas, pressure and/or processing times, for example, in the reduced-pressure carburization step to find conditions in which the carbon concentration at the surface of the first portion is within the specified range. When each steel member to be processed has the same shape, the number of preliminary experiments can be reduced through the use of accumulated data. The first portion and the second portion of the steel member may be determined by actually measuring carbon concentrations at a plurality of positions in the preliminary experiments. However, the first portion and the second portion are relatively easily judged based on their shapes, and thus can be determined through observation of their shapes.

The quenching step is performed as described above by heating a selected portion of the steel member using high-density energy and subsequently subjecting that heated portion to rapid cooling. In this quenching step, not the entire steel member, but only the desired (selected) portion of the steel member (i.e., that portion whose strength is to be improved by quenching) is rapidly heated using the high-density energy and then rapidly cooled. As a result, compared with a case where the entire steel member is subjected to quenching as in the related art, distortion during quenching can be significantly reduced, and the shape before the quenching step of the present invention can be substantially maintained even after quenching.

In this quenching step, high-density energy is used and thus the effect of increasing the strength by quenching can be increased. Furthermore, because improved quenching is obtained, even if the level of carburization in the reduced-pressure carburization step, i.e., the depth of carburization is reduced, the reduced level can be compensated for by the improved quenching capability. The present invention actively uses this superior characteristic so that the carbon concentration of the first portion, resulting from the reduced-pressure carburization step, can be set to 0.65±0.1 weight % after diffusion, which is lower than conventional. More specifically, by combining the quenching step using high-density energy with the above-described reduced-pressure carburization step, the carburization time in the reduced-pressure carburization step can be shortened to provide higher efficiency, and formation of excessive cementite can be reduced to provide improved quality.

The high-density energy may be, for example, a high-density energy beam such as an electron beam or a laser beam, or non-beam high-density energy such as induction heating by high-frequency heating.

On the other hand, even with a quenching step using high-density energy to suppress distortion, if the steel member is distorted before this step, of course, it is difficult to obtain a precisely manufactured steel member. In order to solve such a problem, a slow cooling step that suppresses the distortion of the steel member is interposed between the reduced-pressure carburization step and the quenching step.

As described above, the method of manufacturing a steel member of the present invention includes reduced-pressure carburization in which a relatively small amount of carburization gas is used while maintaining the interior of a carburization furnace at a high temperature and a reduced pressure. Thus, the steel member can be processed with higher efficiency and lower energy than in the related art. Further, as described above, as the carburization condition, the carbon concentration of the first portion is set to 0.65±0.1 weight % after diffusion, which is lower than in the conventional process, and the above-described quenching method is used. Thus, a high quality surface modification is obtained. Furthermore, the use of the slow cooling step provides a precisely manufactured steel member having little distortion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graph of temperature versus time illustrating a heating pattern in an embodiment of the method of the present invention (Example 1);

FIG. 1B illustrates a heat pattern of another method, used for comparison, in Example 1;

FIG. 2A is a schematic diagram of thermal processing equipment used for performing the method of the present invention in Example 1;

FIG. 2B is a schematic diagram of carburizing-quenching equipment used in the comparison method of Example 1;

FIG. 3A is a plan view of the steel member treated in Example 1;

FIG. 3B is a cross-sectional view of the steel member taken along the arrow A-A in FIG. 3A;

FIG. 4 is a graph of hardness versus distance from the surface, illustrating hardness distribution after carburization and quenching in Example 1;

FIG. 5 is a table of values for distortion for the treated articles in Example 1;

FIG. 6 is a graph of residual stress versus distance from the surface for the samples treated in Example 1;

FIG. 7 is a graph of hardness versus carbon content, illustrating the relationship between the carbon concentration at the surface and the surface hardness after quenching in Example 1;

FIG. 8 is a perspective view of another example of a gear treated in Example 1;

FIG. 9 is a perspective view of yet another example of a gear treated in Example 1;

FIG. 10A is a partial perspective view of the details of a tooth section of a gear treated in Example 1;

FIG. 10B is a partial expanded cross-sectional view of a tooth shown in FIG. 10A;

FIG. 10C is a partial expanded view showing the exterior surfaces of the first and second portions of a tooth shown in FIG. 10A.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The reduced-pressure carburization step of the present invention requires that, as a carburization condition, the carbon concentration of a first portion be set to 0.65±0.1 weight % after diffision. When the carbon concentration of the first portion, resulting from the reduced-pressure carbonization step, is lower than 0.55 weight % after diffusion, even when a quenching step is used in which heating with high-density energy is followed by rapid cooling, sufficient hardening may not be obtained in the first portion. On the other hand, when the carbon concentration in the first portion exceeds 0.75 weight % after diffusion, the carbon concentration of the second portion will probably exceed 0.85 weight % after diffusion, causing formation of cementite and reducing the surface modification effect.

Thus, the reduced-pressure carburization step is preferably performed under conditions which give a carbon concentration at the surface of the first portion within a range of 0.65±0.05 weight % after diffusion.

More preferably, the reduced-pressure carburization step is performed under conditions that are obtained in advance through experiment and that give a carbon concentration at the surface of the second portion equal to or lower than 0.85 weight % after diffusion.

In the reduced-pressure carburization step the steel member is exposed to contact with a hydrocarbon-based carburization gas and carbon is diffused inside the steel member at a reduced pressure. In this case, conditions can be set in a relatively easy manner. In particular, the present invention controls a target value for the carbon concentration at the surface of first portion as a process condition. The carbon concentration can be changed by adjusting the carburization period, i.e., the time of contact with the hydrocarbon gas while heating. The specific time must be confirmed through preliminary experiment, which may be easily performed if data for similar types of shapes is available.

The first portion of the steel member is a portion where the angle of the surface, in cross-section of the first portion, is at least 130 degrees. The first portion and the second portion are preferably subjected to reduced-pressure carburization as a preliminary experiment to measure and compare the carbon concentrations of the respective portions to determine optimum conditions. They may be empirically and accurately identified based on their shapes. In particular, as described above, if the comparison of the cross-sectional shapes of the respective parts shows that a certain portion has angle θ at the surface of the cross-section of 130 degrees or more (i.e., the portion is not empirically judged as a protrusion), that portion may be taken as the first (“easy carbon diffusion”) portion. While the side surfaces 811 and the top surface 812 are slightly curved, they approximate planar surfaces.

Further, as shown in FIG. 10B, if the comparison of the cross-sectional shapes of the respective parts shows that a certain portion has an angle θ at the surface of the cross-section of 130 degrees or less, that portion may be taken as the second (“difficult carbon diffusion”) portion.

The angle θ of the second portion, of gears in an exemplary, commercially available automatic transmission have been measured as: 118.15°, 125.7°, 112.7°, 111.5°, 124.8°, 119°, and 113.7°, respectively. It will be recognized that these are exemplary values only, and that the second portion can include any suitable angle θ of 130 degrees or less.

As is illustrated in FIG. 10C, an exterior surface 816 of the first portion, shown by lines and arrows, includes a portion of the top surface between corner sections 813 of the gear teeth. Exterior surfaces of the first portions can also include portions of the tooth bottom 815 and the tooth face 811. As is also shown, an exterior surface 817 of the second portion, shown by lines and arrows, includes the corner section 813.

The first and second portions in the present invention are both portions that are subjected to carburization and the subsequent quenching for surface modification. Therefore, any portion not subjected to quenching should not be regarded as either of first and second portions.

When the steel member is a gear having teeth, the first portion is preferably a tooth face (or tooth faces) or a groove bottom (or bottoms) between teeth. The tooth face and the groove bottom have a gently curved plane relatively close to a flat plane and thus are portions which are treated as first portions in the present invention, i.e. portions into which carbon is more easily diffused, as compared to the second portions. On the other hand, a tooth tip corner section between a tooth tip face and a tooth face has a protruding shape and thus is treated in the present invention as a second portion whose surface is treated to have a higher carbon concentration than the surfaces of the first portions. Experimental results obtained for the present invention show that, when the steel member is a gear, even for steel members of slightly different shapes, by subjecting the steel member to reduced-pressure carburization in such a manner that the tooth faces or the groove bottoms between the teeth have a carbon concentration of 0.65±0.1 weight % after diffusion, the carbon concentration at the corners 813, where the tooth tip faces 812 and the tooth side faces 811 intersect, will be higher than that of the first portions and equal to or lower than 0.85 weight % after diffusion. Thus, a superior surface modification effect is obtained.

The reduced-pressure carburization step is preferably performed while heating the steel member at a temperature equal to or higher than its austenitizing temperature and under a reduced pressure of 1 to 100 hPa. The disadvantage of a reduced pressure during carburization lower than 1 hPa, is that high-cost equipment for maintaining a high degree of vacuum is required. When the reduced pressure during carburization exceeds 100 hPa on the other hand, soot is generated during carburization, thus causing a problem of uneven carbon concentration. The carburization gas may be, for example, a hydrocarbon gas such as acetylene, propane, butane, methane, ethylene, or ethane.

The slow cooling step is preferably performed at a cooling rate less than that at which the steel member transforms to martensite during cooling. Accordingly, distortion can be reduced.

The reduced-pressure carburization step can use high concentration carburization in which the surface carbon concentration is increased to a level higher than in conventional carburization, to precipitate a compound of iron and carbon in the top layer, or carburization nitriding in which carburization and nitriding are simultaneously performed.

The slow cooling step is preferably performed at a cooling rate from 0.1 degree C./second to 3.0 degrees C./second while the temperature of the steel member is equal to or higher than an A1 transformation point temperature. When the cooling rate of the slow cooling step exceeds 3.0 degrees C./second, while the temperature of the steel member is equal to or higher than the A1 transformation point temperature, distortion during cooling may not be sufficiently reduced. On the other hand, when the cooling rate of the slow cooling step is lower than 0.1 degree C./second, while the temperature of the steel member is equal to or higher than the A1 transformation point temperature, a long time is required for reaching the A1 transformation point temperature where carburized carbon is increasingly diffused into the steel member. The progress of diffusion during slow cooling varies depending on the temperatures of the first and second portions, thereby causing variation in diffusion rates and a variation in carbon content.

The cooling gas used in the slow cooling step is preferably nitrogen, helium, argon or a combination thereof. These gases are so-called inert gases and can prevent a steel member from becoming oxidized during slow cooling.

The slow cooling step is preferably performed with the cooling gas at a pressure lower than atmospheric pressure to further suppress distortion during cooling.

When the cooling gas is agitated during cooling, the low-pressure cooling gas having a difference in the cooling rates between the upwind side and the downwind side of the circulating cooling gas is reduced as compared with the case where the cooling gas is at atmospheric pressure. Specifically, when slow cooling is performed at atmospheric pressure, heat exchange is promoted by mere contact between the steel member and the cooling gas. In this latter case, gas convection induced by forced gas agitation or heat creates an upwind side and a downwind side with different cooling rates. The difference in the cooling rates causes a temperature difference in the member undergoing treatment, thus causing distortion due to thermal stress. On the other hand, the cooling gas at the reduced pressure can provide, at both the upwind side and the downwind side, a slow heat exchange rate and thus a difference in the cooling rate is prevented. Thus, the use of the reduced-pressure slow cooling using cooling gas at a reduced pressure (below atmospheric pressure) promotes cooling in a relatively uniform manner and thus it is less likely to cause distortion due to thermal stress. Even without agitation, cooling gas at a reduced pressure can reduce, compared to use of cooling gas at atmospheric pressure, the difference in the cooling rates due to residual cooling gases having different temperatures.

By using a cooling gas at a reduced pressure as described above, the steel member treated in the reduced-pressure slow cooling step undergoes little, if any, distortion and thus can be subjected to the quenching step while maintaining high dimensional accuracy.

By performing the reduced-pressure carburization step at a reduced pressure (vacuum) and the slow cooling step at a reduced pressure continuously, the equipment can be structured with a reduced-pressure carburization chamber directly connected to a slow cooling chamber. Therefore, a preliminary chamber for adjusting the degree of depressurization is not required between the chambers. Specifically, the reduced-pressure carburization step and the slow cooling step are both performed at a reduced pressure and thus a pressure difference therebetween can be kept small. Thus, a product obtained through the reduced-pressure carburization can be subjected to the slow cooling at a low pressure without being subjected to normal pressure and thus processed efficiently while suppressing distortion.

The pressure cooling gas at a reduced pressure used in the slow cooling step is preferably in a range from 100 hPa to 650 hPa. When the cooling gas has a pressure higher than 650 hPa, the effect of the reduced pressure may not be sufficiently obtained. When the cooling gas has a pressure lower than 100 hPa, on the other hand, the slow cooling step may become difficult due to the configuration of equipment. Therefore, the cooling gas used in the slow cooling step more preferably has a reduced pressure in a range from 100 hPa to 300 hPa.

The cooling gas at a reduced pressure used in the slow cooling step is preferably made higher than in its previous state after the temperature of the steel member becomes equal to or lower than the A1 transformation point. In the slow cooling at a reduced pressure, the higher the degree of depressurization, i.e. degree of vacuum, the more distortion is suppressed, but the cooling efficiency is also lowered. However, distortion is avoided when the temperature of the steel member is equal to or lower than the A1 transformation point. Thus, the effect of reducing distortion can be maintained even when the pressure of the cooling gas is increased to increase the cooling efficiency.

Next, in the quenching step a desired (selected) portion of the cooled steel member is heated using high-density energy and subsequently rapidly cooled. By heating the steel member using high-density energy to a temperature equal to or higher than its austenitizing temperature as described above, localized heating can be easily realized and thus the effect of reducing distortion can be significantly improved as compared with heating of the entire steel member.

The cooling rate for the rapid cooling is desirably in a range from 200 degrees C./second to 2000 degrees C./second. When the cooling rate is lower than 200 degrees C./second, a quenching effect may not be sufficiently obtained. On the other hand, a cooling rate exceeding 2000 degrees C./second makes it difficult to realize rapid cooling. Pressure is not particularly important in the rapid cooling.

It is important that the quenching step be performed by heating the selected portion of the steel member using high-density energy at a temperature equal to or higher than its austenitizing temperature and subsequently rapidly cooling the steel member at a cooling rate equal to or higher than a critical cooling rate for rapid cooling at which martensitic transformation occurs in a carburization layer. Accordingly, a sufficient hardening in the carburization layer can be obtained.

The heating using high-density energy in the quenching step is preferably high-frequency heating and the rapid cooling is preferably by water quenching. The high-frequency heating can be induction heating in a non-contact manner with a higher efficiency and accuracy.

The rapid cooling when induction heating by high-frequency is used is preferably by water quenching. The use of the induction heating by high-frequency allows a portion of a steel member to be selectively heated. In other words, the steel member is partially, not entirely, heated. Thus, even when water quenching is subsequently performed using water providing a very high cooling rate, quenching distortion can be minimized. The superior rapid cooling effect with water quenching can improve quenching characteristics, and thus a quenched part can be imparted with yet higher strength. Thus, a required strength can be attained even when carburization is simplified (i.e., the processing time is shortened), which means a carburization layer of reduced thickness. In this case, the time required for the entire thermal processing step can be further shortened.

Preferably, steel members are subjected one by one to induction heating by high-frequency and the heated members subsequently cooled by spraying cooling water directly toward the steel members while rotating the steel members. In this case, the steel members can be uniformly cooled and thus distortion can be further suppressed.

The quenching step can also be performed by focusing a high-density energy beam onto the selected portion of the steel member to selectively heat that portion and then rapidly cooling the steel member, e.g. by water quenching or self cooling. Specifically, a high-density energy beam, e.g. an electron beam or a laser beam, can be used to extremely rapidly heat a selected surface area on which the beam is focused. By limiting the heated portion to the outer (exterior) surface, a sufficient rapid cooling effect can be obtained by self cooling when the heating with the high-density energy beam is stopped or the steel member is removed from the heating chamber.

The high-density energy beam is preferably an electron beam. Electron beams can be easily changed to have a different output, a different beam diameters and/or a different emission region, for example. Thus, the desired region can be selectively heated with high accuracy.

When an electron beam is used, to the selected portion on which the beam impinges can also be melted rapidly. Thus, the quenching step is preferably performed by emitting an electron beam onto a desired portion of the steel member to heat only an outer layer to a temperature equal to or higher than the melting point to form a melt, and by subsequently rapidly cooling the melted portion until reaching a martensitic transformation region, to obtain a martensite structure and to form a hardened layer.

The hardened (outer) layer preferably has a thickness of 0.2 mm or less. A hardened layer having a thickness exceeding 0.2 mm may be an obstacle to self cooling after melting. A hardened layer that is too thin on the other hand may result in reduced durability. Thus, the hardened layer more preferably has a thickness in a range from 0.1 mm to 0.2 mm.

The steel member to which the method of the present invention is applied may be a component of an automobile drive system (drive train). Components for an automobile drive system include, for example, a gear, a ring-shaped member, and other components of an automatic transmission. They are components that require both high strength and high dimensional accuracy. Thus, by using the above-described superior thermal processing method, the manufacture can be streamlined with low cost and the resultant products have high quality.

EXAMPLES Example 1

An example of the method of manufacturing a steel member according the present invention will now be described with reference to the accompanying drawings. In this example, a manufacturing method in accordance with the present invention and a related-art manufacturing method for comparison (comparative example) were applied to a ring-shaped steel member 8 (ring gear), used as a component of an automatic transmission, to evaluate the surface hardness and the distortion. In this example, the steel member 8 to be treated included, as shown in FIG. 8 (FIG. 3 is a schematic view of FIG. 8), a tooth section 81 provided at the inner circumferential surface of a ring-like body 80. The steel member 8 is a component in which the tooth section 81 has high hardness, and for which roundness is very important.

FIG. 1 shows a heat pattern A of the method of the present invention and, for purposes of comparison, a heat pattern B (comparative example). In FIG. 1, the horizontal axis shows time and the vertical axis shows temperature, and the temperatures of the steel members during thermal processing are shown as the heat patterns A and B.

As can be seen from the heat pattern A of FIG. 1, the method of the present invention starts with a reduced-pressure carburization step a1 for heating a steel member to a carburization temperature of 950 degrees C. and retains the steel member at that temperature for 49 minutes. Thereafter, the steel member is subjected to a reduced-pressure slow cooling step a2 by which the steel member is cooled down to 150 degrees C. or less for 40 minutes. Thereafter, the steel member is subjected, in a high-frequency quenching step a3, to induction heating by high-frequency to rapidly heat the member up to a quenching temperature of 950 degrees C. and is subsequently subjected to water quenching.

On the other hand, as is seen in the heat pattern B, the comparative method starts with a conventional carburization step b1 to heat a steel member to a carburization temperature of 950 degrees C. and retains the member at that temperature for 220 minutes. Thereafter, the member is subjected to a quenching step b2 in which the member is maintained at a quenching temperature of 850 degrees C. and is subsequently subjected to oil quenching. In the comparative example, a post-washing step b3 for washing off coolant (oil) which the steel member retained from oil quenching and a tempering step b4 for securing the toughness of the hardened layer are performed, during which the temperature of the member is increased to some degree. Distortion evaluation, strength evaluation, and residual stress evaluation, which will be described later, were performed after this tempering step b4.

Before describing details of these processes, the thermal processing equipment 5 for carrying out the method of the present invention and carburizing-quenching equipment 9 for carrying out the comparative example will be briefly described.

As shown in FIG. 2A, the thermal processing equipment 5 for carrying out the method of the present invention includes: a reduced-pressure carburization slow cooling apparatus 52 that includes a pre-washing bath 51 for washing a steel member before carburization and quenching, a heating chamber 521, a reduced-pressure carburization chamber 522, a reduced-pressure slow cooling chamber 523; a high-frequency quenching machine 53 and a magnaflux apparatus 54 for inspecting for defects.

As shown in FIG. 2B, the carburizing-quenching apparatus 9 for carrying out the comparative example included: a pre-washing bath 91 for washing the steel member before carburization and quenching; a large elongated carburization furnace 92 that includes a carburization furnace 921 for performing heating, carburization, and diffusion, and a quenching oil bath 922; a post-washing bath 93 for washing the steel member after carburization and quenching; and a tempering furnace 94 for tempering.

Next, steel members 8 were subjected to carburization and quenching in the apparatus of FIG. 1A and FIG. 1B, respectively. Then, the steel members 8 were compared with regard to strength, distortion, and residual stress.

In the method of the present invention, as shown in the heat pattern A of FIG. 1, the steel member was subjected to reduced-pressure carburization in step a1 to carburize the steel member placed in carburization gas under a reduced pressure, to reduced-pressure slow cooling in step a2 to cool the steel member in cooling gas at a pressure lower than atmospheric pressure, and to the high-frequency quenching step a3 in which a desired portion of the cooled steel member was subjected to induction heating by high-frequency and then to water quenching.

The reduced-pressure carburization step a1 has a carburization period during which the steel member is maintained in the hydrocarbon carburization gas and carbon enters (diffuses) into the surface of the steel member, and a diffusion period during which carbon diffuses into the interior of the steel member. In this example, both of these processes (the carburization and diffusion processes) were conducted by retaining the steel member for 49 minutes at 950 degrees C. which is a temperature equal to or higher than its austenitizing temperature. These processes were performed under conditions with the carburization chamber drawn down to a pressure of 1 hPa and wherein the carburization gas was acetylene. In other words, both of the carburization period and the diffusion period were at a reduced pressure as described above, and acetylene was introduced to the carburization chamber during the carburization period, while the introduction of acetylene was stopped and only a reduced pressure was applied during the diffusion period. The temperature was fixed as described above during the carburization period and the diffusion period. Note that the steel member 8 is a ring gear. Due to the shape of the ring gear, the steel member 8 has: a first (“easy carbon diffusion”) portion in which, during carburization, the carbon has a high diffusion rate, which first portion is composed of a tooth bottom 815 and a tooth face 811; and a second (“difficult carbon diffusion”) portion in which, during carburization, the carbon diffusion rate is lower than that in the first portion and which is composed of a tooth tip corner section 813 (a corner section between the tooth face 811 and a tooth tip 812). The reduced-pressure carburization step is performed under conditions which give the tooth bottom 815 a carbon concentration at the surface within a range of 0.65±0.05 weight % after diffusion. As is shown in FIG. 10A, the tooth bottom 815 and the tooth face 811 have surface angles in the cross-sectional shapes that are close to 180 degrees and are thus identified as portions having an angle equal to or greater than 130 degrees.

In the reduced-pressure slow cooling step a2 the cooling gas was nitrogen N₂), the reduced pressure was 200 hPa, the cooling gas was agitated, the reduced-pressure slow cooling step was performed during a period extending from a temperature just after carburization equal to or higher than its austenitizing temperature to a temperature of 150 degrees C. lower than the A1 transformation point, and the cooling rate was within a range from 0.1 to 3.0 degrees C./second (specifically, 10 degrees C./minute (0.17 degrees C./second)).

In the high-frequency quenching step a3 the tooth section 81 on the inner circumferential surface of the steel member 8 was subjected to induction heating by high-frequency to a temperature of 950 degrees C., i.e. to a temperature equal to or higher than its austenitizing temperature, and subsequently subjected to water quenching where water was sprayed onto the tooth section 81 to provide a cooling rate equal to or higher than the critical cooling rate for rapid cooling, resulting in martensitic transformation in a carburized layer. This cooling rate by water quenching was 268 degrees C./second. The induction heating by high-frequency was by a method in which the steel members 8 were transported one by one to heat the individual steel members, and were subjected to cooling after the heating by cooling water sprayed directly onto the steel members 8 while the steel members 8 were rotated to cool them, one by one to minimize distortion.

In the comparative example, as is seen in the heat pattern B of FIG. 1, a steel member was heated to the carburization temperature of 950 degrees C. and retained at that temperature for 220 minutes, and subsequently subjected to the conventional carburization step b1. Thereafter, the member was retained at the quenching temperature of 850 degrees C., and subsequently subjected to the quenching step b2 with oil quenching. The carburization in the comparative example was performed under conditions in which carbon potential was adjusted to give substantially the entire surface of the steel member 8 a carbon concentration of 0.8 weight % after diffusion. In the comparative example, the quenching step b2 was followed by the post-washing step and the post-washing step b3 was followed by the tempering step b4.

Both the comparative example and the example representative of the method of the present invention used SCM420 (JIS) steel which is suitable for carburization.

The steel member obtained through carburization and quenching was tested to determine Vickers hardness (Hv) with distance from the surface at the tooth bottom 815 of the gear (FIG. 3) and the measured Vickers hardness was used to evaluate strength. The results are shown in FIG. 4. In FIG. 4, the horizontal axis represents the distance from the surface (mm) and the vertical axis represents the measured Vickers hardness (Hv). The results for the steel member treated by the method of the present invention are shown by reference numeral E1 while the results for the steel member obtained by the comparative example are shown by a reference numeral C1.

As seen in FIG. 4, the method of the present invention (E1), results in a hardness which is slightly reduced at a given distance from the surface as compared to the hardness for the comparative example (C1). However, the hardness at the outer surface is higher for the method of the present invention as compared to that obtained with the comparative example. As can be seen from these results, the method of the present invention provides superior thermal processing equal to or higher than the related art. In particular, by the method of the present invention, a tooth bottom section 815 had a carbon concentration (0.65±0.05 weight % after diffusion) that was lower than the carbon concentration there in the comparative example (0.8 weight % after diffusion); but sufficient quenching was obtained.

Furthermore, the measurement of the carbon concentration at the surface of the tooth tip corner section 813 of the steel member 8, obtained in the method of the present invention, was 0.8 weight % after diffusion, and the hardness thereof was equal to that of the tooth bottom section 815. These results demonstrate the effectiveness of the reduced-pressure carburization step in the method of the present invention.

In FIG. 7, the horizontal axis represents the carbon concentration (content) at the surface, and the vertical axis represents the surface hardness after quenching. In FIG. 7, curve A shows the relationship between the surface carbon concentration obtained in the reduced-pressure carburization step of the method of the present invention and hardness after water quenching, which relationship was experimentally determined, and curve B shows the relationship between the carbon concentration and the hardness after oil quenching in the carburization step of the related-art method that was also experimentally obtained. A region S (of a carbon concentration of 0.85% or more after diffusion) is also shown where an abnormal structure tends to be generated due to excessive carburization.

In FIG. 7, with regard to the actual carbon concentration finally reached when carburization conditions for a carbon concentration of 0.8 weight % after diffusion are selected for the related-art method, the first (“easy carbon diffusion”) portion (the tooth bottom 815) is shown by the position of the tip end of an arrow b1 and the second (“difficult carbon diffusion”) portion (the tooth tip corner section 813) is shown by the position of the tip end of an arrow b2.

With regard to the carburization concentration finally reached when carbon conditions for a carbon concentration of 0.8 weight % after diffusion of the carbon easy diffusion portion (the tooth bottom 815) are selected for the method of the present invention, the first (“easy carbon diffusion”) portion (the tooth bottom 815) is shown by the position of the tip end of an arrow cl and the second (“difficult carbon diffusion”) portion (the tooth tip corner section 813) is shown by the position of the tip end of the arrow c2.

With regard to the carbon concentration finally reached when conditions for a carbon concentration of 0.60 weight % after diffusion at the surface of the first (“easy carbon diffusion”) portion (the tooth bottom 815) are selected for the method of the present invention, the first (“easy carbon diffusion”) portion (the tooth bottom 815) is shown by the position of a tip end of an arrow a1 and the second (“difficult carbon diffusion”) portion (the tooth tip corner section 813) is shown by the position of a tip end of the arrow a2.

As is shown in FIG. 7, by the related-art method, the hardness after quenching becomes higher as the carbon concentration increases. On the other hand, excessive carburization is problematic and thus the carbon concentrations of all portions are desirably set to be 0.8 weight % after diffusion. With regard to this point, the related-art method can provide the first and second portions both having substantially the same carbon concentration. Thus, the carbon concentration conditions can be set to 0.8 weight % after diffusion to provide high hardness for the entire article.

By the method of the present invention, the hardness after quenching is substantially the same in a range of carbon concentration from 0.6 to 0.8 weight % after diffusion. This is a result of using the above-described superior quenching step.

When the reduced-pressure carburization was performed so that the first portion (the tooth bottom 815) had a carbon concentration of 0.8 weight % after diffusion (c1), a carbon concentration (c2) of the second portion (the tooth tip corner section 813) entered the excessive carbon region S.

On the other hand, when the reduced-pressure carburization was performed so that the first portion (the tooth bottom 815) had a carbon concentration of 0.6 weight % after diffusion (a1), the carbon concentration (a2) of the second portion (the tooth tip corner section 813) was reduced to a range equal to or lower than 0.8 weight % after diffusion. Even when there was a difference in the carbon concentration as described above, the resultant hardness can be maintained at substantially the same level, as can be seen from FIG. 7.

When the steel material suitable for carburization as used in the related-art method was used in the method of the present invention (E1), it might be expected that a significantly-reduced carburization time would result in a proportionally-reduced carburization depth and reduced strength. However, these potential strength-related disadvantages were overcome by changing the material used and by using water quenching. There is also the possibility that the interior strength can be improved to a level equal to that of the related-art product by selection of a material of appropriate composition.

Next, the sizes of the steel members obtained by the carburization and quenching treatment were measured to compare the degrees of distortion. The sizes of the steel members were measured based on two types of shapes, i.e. “BBD” and “BBD ellipse”. As shown in FIG. 3, the “BBD” indicates a size in which steel balls 88 having a predetermined diameter are placed so as to abut valleys in the tooth face 81 and an inner diameter between the opposing hard balls 88 is measured. This measurement was performed at three positions in the axial direction (positions a, b, and c in FIG. 3B) for the entire circumference, and an average value (Ave), the maximum value (Max), and the minimum value (Min) of the measurements were obtained.

Next, the difference between the maximum value and the minimum value of the “BBD” at the respective measured positions in the axial direction was calculated as “BBD ellipse (μm)”. Then, as above, an average value (Ave), the maximum value (Max), and the minimum value (Min) of the measurements were obtained.

FIG. 5 illustrates the results with the “BBD” and “BBD ellipse”. In FIG. 5, the left column shows the results of the method of the present invention at three different points in time, i.e. (1) “before reduced-pressure carburization”, (2) “after low-pressure carburization and reduced-pressure slow cooling”, and (3) “after high-frequency quenching”. In FIG. 5, the right column shows the results for the comparative example “before carburization and quenching” and “after carburization and quenching”. The lines shown in the respective columns are obtained by plotting the maximum values, the minimum values, and the average values at the positions a, b, and c in FIG. 3B, respectively, and connecting each of the maximum values to each of the minimum values by a thick line. Average values at the three positions are connected by a thin line.

As seen in FIG. 5, the method of the present invention suppresses the distortion even after quenching. Further, it can be seen that suppression of distortion has already been obtained by the reduced-pressure slow cooling after the reduced-pressure carburization. In contrast, it can be seen that significant distortion is caused by carburization and quenching in the comparative example.

Next, residual stresses of the steel members after carburization and quenching were measured and were compared. The measurements are shown in FIG. 6. In FIG. 6, the horizontal axis represents distance from the surface of the tooth bottom 815, and the vertical axis represents residual stress by showing a tensile direction by the mark + and a compression direction by the mark −.

By the method of the present invention (El), compressive residual stress is found at least at the outermost surface. In the comparative example (C1) on the other hand, tensile residual stress is found at the outermost surface. When the residual stress at the outermost surface is tensile stress, various problems may arise. Thus, thermal processing or surface modification processing is required to reduce the tensile residual stress. Thus, the method of the present invention provides another advantage in that a process for improving such residual stress is not particularly required.

The advantages of the present invention are not limited to a ring gear but can be obtained when the method is applied to various other gears with teeth. As examples of such gears, the above-described ring gear 8 is shown in FIG. 8, and a worm wheel 802 with external teeth is shown in FIG. 9. FIG. 10A is an exploded view illustrating the tooth section 81 in the external tooth gear and shows the tooth section 81 as having a tooth bottom 815, a tooth face 811, a tooth tip 812, and a tooth tip corner section 813. When the treatment method of the present invention is applied to a gear as described above, conditions of a reduced-pressure carburization step may be set so that the tooth face 811 and/or the tooth bottom 815 is given a carbon concentration of 0.65±0.1 weight % after diffusion.

Example 2

In this example, the method of the present invention as in Example 1 was applied to a steel member 8 (ring gear) having the same shape and material as in Example 1 but with a different target value for the carbon concentration at the surface of the tooth face 811. The tooth tip corner section 813 and the tooth face 811 of the resultant tooth section 81 were measured for carbon concentration and surface hardness, and the surface structures of the both were observed. The target value of the carbon concentration of the surface of the tooth face 811 in the reduced-pressure carbon step was set, as shown in Table 2, to be 0.65 weight % after diffusion in Example 1 of the present invention, 0.57 weight % after diffusion in Example 2 of the present invention, and 0.75 weight % after diffusion in Example 3 of the present invention.

For comparison, steel members were also treated by two comparative methods.

In comparative Example 1 the target value of the carbon concentration at the surface of the tooth face 811 in the reduced-pressure carburization step was increased to 0.78 weight % after diffusion as shown in Table 2.

Comparative Example 2 is a method in which oil quenching is performed just after related-art gas carburization as in comparative Example 1. In this case, the target value for the carbon concentration at the surface of the tooth face 811 and the tooth tip corner section 813 in the gas carburization step was set to 0.75 weight % after diffusion as shown in Table 2.

Table 1 and Table 2 show the conditions and results for Examples 1 to 3 of the present invention and Comparative Examples 1 and 2.

TABLE 1 Carburization/ Cooling After Cooling Rate Diffusion Carburization/ After Test No. Method Component Material Carburization Temperature Diffusion Carburization/Diffusion Case Present Ring Gear SCM42H Vacuum 950° C. Gas cooling 0.16° C./sec. to Example 1 Invention carburization 1.8° C./sec. (at applied 600° C. or higher) Case Present Ring Gear SCM42H Vacuum 950° C. Gas cooling 0.16° C./sec. to Example 2 Invention carburization 1.8° C./sec. (at applied 600° C. or higher) Case Present Ring Gear SCM42H Vacuum 950° C. Gas cooling 0.16° C./sec. to Example 3 Invention carburization 1.8° C./sec. (at applied 600° C. or higher) Comparison No use of Ring Gear SCM42H Vacuum 950° C. Gas cooling 0.16° C./sec. to Example 1 target values carburization 1.8° C./sec. (at in low 600° C. or pressure higher) carburization Comparison Comparison Ring Gear SCM42H Gas 950° C. Oil cooling Rapid Cooling Example 2 method carburization (110° C./sec.) (related-art gas carburization)

TABLE 2 Carburization Target Concentration carburization After Diffusion concentration (mass %) Surface Hardness Surface structure (presence value after Tooth tip Tooth tip or non-presence of Reheating/ diffusion (tooth corner Tooth corner Tooth abnormal structure) Test No. Quenching face, mass %) section face section face Tooth tip Tooth face Case High 0.65 0.74 0.65 Hv820 Hv810 Martensite Martensite Example 1 frequency heating + Water quenching Case High 0.57 0.71 0.57 Hv818 Hv787 Martensite Martensite Example 2 frequency heating + Water quenching Case High 0.75 0.84 0.74 Hv825 Hv812 Martensite Martensite Example 3 frequency heating + Water quenching Comparison High 0.78 0.91 0.78 Hv823 Hv805 Cement Martensite Example 1 frequency precipitation heating + (abnormality) Water quenching Comparison Not applied 0.75 0.76 0.73 Hv780 Hv765 Martensite Martensite Example 2

As can be seen in Table 1 and Table 2, steel members (ring gears) obtained by the method of the present invention in Examples 1 to 3 show that both the tooth tip corner section 813 and the tooth face 811 had carbon concentrations within a range from 0.65 to 0.85 weight % after diffusion and extremely superior hardness, and that the structure was a sound martensite structure with no cementite precipitation.

In Comparative Example 1, on the other hand, the target value for the carbon concentration at the surface of the tooth face 811 of the first (“easy carbon diffusion”) portion in the reduced-pressure carbon step was set to a value exceeding 0.75 weight % (0.78) after diffusion. Thus, the actual value of the carbon concentration at the surface of the tooth face corner section 813, which served as the second portion, exceeded 0.85 weight % and reached 0.91 weight % after diffusion, resulting in an abnormal structure with cementite precipitation.

In Comparative Example 2, the surface hardness was slightly lower than in examples 1-3 of the present invention. This clearly shows that, so long as the carbon concentration is in a range from 0.55 to 0.85 weight % after diffusion, the present invention provides superior results in terms of strength characteristics and hardness.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 

1. A method of carburizing-quenching a steel member, comprising: (a) a reduced-pressure carburization step of carburizing a steel member in a carburization gas at a pressure below atmospheric pressure giving a first portion of the steel member a carbon concentration at 0.65±0.1 weight % after diffusion at the exterior surface of the first portion; (b) a slow cooling step of subjecting the steel member, treated in step (a) to have a carbon concentration of 0.65±0.1 weight % after diffusion at the exterior surface, to a first rate of cooling in a cooling gas; and (c) subsequent to step (b), a quenching step of selectively heating a selected portion of the cooled steel member using high-density energy and subsequently subjecting the selected portion to a second rate of cooling, more rapid than the first rate of cooling, wherein the first portion of the steel member subjected to the reduced-pressure carburization step (a) has an exterior surface contoured in a manner facilitating the introduction of carbon by diffusion during the reduced-pressure carburization step and wherein the steel member subjected to the reduced-pressure carburization step (a) also has a second portion with an exterior surface shaped to have a carbon diffusion rate less than the carbon diffusion rate of the first portion.
 2. The method according to claim 1, wherein: the pressure in step (a) is 1 hPa to 100 hPa; step (a) includes heating the steel member to at least its austenitizing temperature while the steel member is in contact with the carburization gas; the slow rate of cooling in step (b) is from 0.1 degrees C./second to 3.0 degrees C./second and at a pressure of 100 hPa to 650 hPa; the heating of the selected portion in step (c) by impinging an electron beam on the selected portion; the rapid cooling in step (c) is 200 degrees C./second to 2000 degrees C./second; the first portion has an external surface contour with surface portions meeting at an angle of 130 degrees to 180 degrees.
 3. The method according to claim 2 wherein the steel member is a gear having a circumferential surface with gear teeth arranged spaced thereon, each gear tooth having a flat top surface and side surfaces meeting the top surface to form corners and wherein the exterior surface of the first portion includes a portion of the top surface between the corners and the exterior surface of the second portion includes the corner.
 4. The method according to claim 1, further comprising, subjecting other steel members of the same material to step (a) wherein at least one process parameter is varied between different tests to determine a value for the at least one process parameter resulting in a carbon concentration of 0.65±0.01 weight % after diffusion at the surface of the first portion and subsequently using the at least one process parameter having the determined value in step (a).
 5. The method according to claim 4 wherein the at least one process parameter is time of contact of the steel member with the carburization gas in step (a).
 6. The method according to claim 4 wherein the at least one process parameter is selected from temperature, type of carburization gas, pressure, processing time and combinations thereof.
 7. The method according to claim 1, wherein the reduced-pressure carburization step (a) is performed under a condition which gives a carbon concentration at the surface of the first portion in a range of 0.65±0.05 weight % after diffusion.
 8. The method according to claim 1, wherein the reduced-pressure carburization step (a) is performed under a condition which gives a carbon concentration at the surface of the second portion of 0.85 weight % after diffusion or lower.
 9. The method according to claim 1, wherein the reduced-pressure carburization step (a) has a carburization period during which the steel member is in contact with a hydrocarbon carburization gas and carbon is introduced at a surface of the steel member, and a diffusion period during which carbon is diffused inside the steel member no longer in contact with a carburization gas.
 10. The method according to claim 1, wherein the first portion of the steel member is any portion having a surface with an angle in cross-section of 130 degrees to 180 degrees.
 11. The method according to claim 1, wherein the steel member is a gear having a tooth section, and the first portion is at least one of a tooth face and a tooth bottom of the tooth section.
 12. The method according to claim 1, wherein the reduced-pressure carburization step (a) is at a temperature at least as high as the austenitizing temperature of the steel member and at a pressure of 1 to 100 hPa.
 13. The method according to claim 1, wherein the slow cooling step is performed at a cooling rate at which the steel member does not transform to martensite during cooling.
 14. The method according to claim 1, wherein the slow cooling step is performed at a cooling rate from 0.1 degree C./second to 3.0 degrees C./second while the temperature of the steel member is equal to or higher than an A1 transformation point temperature.
 15. The method according to claim 1, wherein the cooling gas used in the slow cooling step (b) is selected from the group consisting of nitrogen, helium, argon, and combinations thereof.
 16. The method of manufacturing a steel member according to claim 1, wherein the slow cooling step is performed by contacting the steel member with cooling gas having a reduced pressure lower than atmospheric pressure.
 17. The method according to claim 16, wherein the reduced pressure of the cooling gas used in the slow cooling step is in a range from 100 hPa to 650 hPa.
 18. The method according to claim 16, wherein the reduced pressure of the cooling gas used in the slow cooling step is in a range from 100 hPa to 300 hPa.
 19. The method according to claim 16, wherein the reduced pressure of the cooling gas used in the slow cooling step is raised after the temperature of the steel member becomes equal to or lower than the A1 transformation point.
 20. The method according to claim 1, wherein the quenching step is performed by heating a selected portion of the steel member, using high-density energy, to a temperature equal to or higher than the austenitizing temperature of the steel member and subsequently rapidly cooling the steel member at a cooling rate equal to or higher than a critical cooling rate, for rapid cooling at which martensitic transformation occurs in a carburization layer.
 21. The method according claim 1, wherein the quenching step is performed by induction heating by high frequency a selected portion of the steel member to heat the selected portion and then rapidly cooling the steel member by water quenching.
 22. The method according to claim 21, wherein the high-density energy is applied to a succession of steel members, one by one, and subsequently cooling the heated steel members by spraying cooling water onto each steel member while rotating the steel member.
 23. The method according to claim 1, wherein the quenching step is performed by emitting a high-density energy beam onto a selected portion of the steel member to heat the selected portion and then rapidly cooling the steel member by self-cooling.
 24. The method according to claim 23, wherein the high-density energy beam is an electron beam.
 25. The method according to claim 13, wherein the second rate of cooling is performed at a cooling rate at which the steel member transforms to martensite during cooling.
 26. The method according to claim 1, wherein the second rate of cooling is performed at a cooling rate at which the steel member transforms to martensite during cooling. 